Effective emission control of internal combustion engine exhaust gases with a catalytic converter requires precise control of the air/fuel ratio supplied to the engine cylinders. For this purpose, it is customary to install an oxygen sensor in the engine exhaust pipe, and to use the sensor output as a feedback signal for closed-loop fuel control.
In general, two different types of oxygen sensors are available for usage in automotive fuel control. The most common and least expensive sensor, referred to as a switching sensor, has a bi-stable output voltage that switches or toggles between first and second states corresponding to lean and rich conditions of the sensed exhaust gas, relative to a stoichiometric air/fuel ratio of approximately 14.7:1 for pump gasoline. The other type of oxygen sensor, referred to as a universal exhaust gas oxygen sensor, or wide-range oxygen sensor, has an analog output that varies in amplitude in relation to the deviation of the sensed exhaust gas from the stoichiometric air/fuel ratio. Of the two sensor types, the switching sensor is less expensive but provides limited information, whereas the wide-range sensor enables improved fuel control performance.
A portion of cost of the wide-range sensor is related to calibration; typically, the calibration is performed off-line by the sensor manufacturer, and active or passive elements in the sensor wiring harness are selected or programmed to provide a standard output voltage vs. air/fuel ratio transfer function. A related disadvantage of the wide-range sensor is that the sensor characteristics may tend to drift with age and/or other factors, leading to fuel control errors since the one-time calibration cannot account for the drift.